1. Field of the Invention
The present invention relates to cooling systems for semiconductor devices and more particularly to a closed loop microchannel cooling system for relatively high power transistor RF amplifier modules.
2. Description of Related Art
In high power, solid state RF amplifier modules, the maximum output power that can be produced by the module is limited by the power density of the active devices, typically transistors. The limits imposed by other components neighboring on the power producing devices are insignificant.
The power density of a transistor is limited by the allowable maximum transition junction temperature. In silicon based transistors, for example, junction temperatures exceeding 125.degree. C. result in a significant decrease in transistor life. The bulk of the temperature rise between ambient and the transistor junction is within the RF transistor itself. The temperature rises due largely to numerous conduction losses and the thermal resistance of the bonds between the transistor components. The heat generated in a transistor is typically conducted through the ceramic and metallic case enclosing the active material. The density of the active cells in their placement on the semiconductor is therefore largely dictated by thermal constraints.
As a consequence, state of the art electronic packaging frequently utilizes redundant parts. This results in increased system costs, size and weight. For example, in an air cooled solid state transmitter, used, for example, in the ARSR-4 air traffic control radar system currently being manufactured by the assignee of this invention, the RF power transistors utilized therein are designed to operate below their power output capability, and are spaced inches apart from one another in order to keep them cooled to military standard operating temperatures. The number of transistors and related peripheral circuits required in such solid state transmitters is dictated by transistor packaging and conventional heat sinks. Accordingly, higher power RF transistors are being developed and used in such systems to enhance their capabilities and increasing reliability, while at the same time reducing size and costs.
It has been demonstrated that by eliminating the thermal interfaces and extracting the heat directly from the active material, the power density of the transistor can be increased by a factor of 2 or more. Doubling the output power of each transistor halves the number of transistors and circuits required and therefore significantly reduces the cost, size and weight of the transmitter.
This has been accomplished, for example in the above cross referenced related applications, U.S. Ser. No. 08/681,207, entitled "Microchannel Cooling Of High Power Semiconductor Devices", and U.S. Ser. No. 08/970,385, entitled "Closed Loop Liquid Cooling Within RF Modules". There cooling is accomplished by forming small coolant passages, called microchannels, in the base of the active material, and passing the coolant into and out of the transistor case through small passages. A pump is used to circulate cooling liquid through the microchannels. Suction and discharge valves are also associated with the pump and typically are located at the pump or in the coolant lines.